1. Field of the Invention
The present invention generally relates to television tuner circuits, and more specifically to a direct conversion television tuner for processing broadcast, cable, and satellite television signals.
2. Related Art
Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHZ, with 6 MHZ channel spacings. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.
Regardless of the transmission scheme, a tuner is utilized to down-convert the received RF signal to an intermediate frequency (IF) signal or a baseband signal, which is suitable for processing and display on a TV or computer screen. The tuner should provide sufficient image rejection during down-conversion as is necessary for the specific application. The National Television Standards Committee (NTSC) sets standards for television signal transmission, reception, and display. To process a NTSC signal, it is preferable that the tuner have a high-level of image rejection. However, less image rejection is acceptable depending on the specific application and the corresponding display requirements.
To achieve a high level of image rejection, traditional TV tuners utilize a dual-conversion architecture having two mixers and a surface acoustic wave (SAW) filter. The first mixer up-converts the received RF signal to a first IF frequency (e.g. 1200 MHZ) that is fixed above the RF signal band, using a variable local oscillator (LO) signal. A SAW filter, centered at the first IF, selects the channel of interest and provides the necessary image rejection to prevent signal interference. The second mixer then down-converts the first IF to a lower frequency second IF, using a second fixed frequency LO signal. The second IF output is at baseband for a NTSC compatible signal. Alternatively, the second IF is at 36 or 44 MHZ for a cable system output that is fed into a set-top box or a cable modem. Channel selection is realized by adjusting the first LO signal so that the desired down-converted channel(s) falls in the narrow passband of the SAW filter. The remaining channels are rejected by the SAW filter.
Due to advances in silicon integrated circuit (IC) technology, most of the tuner components (i.e. mixers, local oscillators, etc.) can be fabricated on a single silicon IC, with the exception of the SAW filter. The SAW filter is a mechanically resonant device that is typically fabricated on a ceramic substrate, and therefore cannot be integrated on-chip with the other tuner components. As such, the SAW device remains a discrete component in the TV tuner design, which prevents the TV tuner from being fabricated on a single silicon substrate.
A single chip solution is highly desirable for TV tuners and cable modems. The single chip solution will reduce or eliminate component adjustment during manufacturing, and therefore lead to a reduction in manufacturing time and cost. The single chip solution will likely improve electrical performance of the tuner because there are parasitics associated with driving a signal off-chip for processing. Additionally, the single chip solution will reduce the size of the tuners, which becomes more critical for non-TV set applications. Therefore, what is required is a TV tuner architecture that can be implemented on a single semiconductor substrate.
Furthermore, a TV tuner with low power requirements is also desired. Cable TV operators plan to offer voice telephone service and/or Internet access to their customers over currently installed cable lines. The telephone traffic will be transmitted over the cable lines at RF frequencies, along with the TV programming. As such, the TV tuner will likely be utilized to down-convert the telephone traffic to baseband for user consumption. In the current POTS (plain old telephone service) system, government regulations require that the phone companies supply sufficient power over telephone lines to operate the customer's telephone. This allows the customer to utilize the telephone during a power outage, to call the electric company for example. If a similar requirement were placed on voice-over-cable service, then the cable company would have to supply enough power over the cable lines to operate the TV tuner.
The conventional dual conversion tuner architecture requires a few watts of power for operation, which is a relatively high power requirement. The high power drain occurs because the dual conversion scheme requires two mixers that are driven by two local oscillators operating at RF frequencies. If voice-over-cable is to be realizable, then the tuner power requirements should be reduced to approximately 0.5 watt of power dissipation.
Therefore, what is needed is a tuner architecture that has good electrical characteristics (e.g. high image rejection, and good linearity), low power requirements, and which can be implemented on a single semiconductor substrate.